This invention relates to the art of prosthetic devices and, more particularly, to the treatment of metal or metal portions of prosthetic devices, and devices resulting from such treatment.
It is of course well known that metal prosthetic devices or such devices including exposed metallic surfaces have been provided for applications including, for example, orthopedic surgery and dentistry in human and or veterinary medicine. Such prosthetic devices include, for example, artificial joints, artificial teeth, special reconstructive surgery devices including plates and screws, and the like. The devices, or the metallic portions thereof, are constructed of metals or metal alloys which are not corroded or otherwise degraded by body fluids. A fundamental obstacle to the long term success or metal prosthetic implants is the permanent fixation of the prosthetic device to the bone in which it is received. Heretofore, it has not been possible to achieve a good bond directly between bone and metal, whereby considerable effort has been made to develop special bonding techniques.
At the present time, prosthetic implants are secured in place in the bone primarily through the use of cements and adhesives. Hip and knee joint replacements are common surgical procedures today in which such adhesive bonding is used, and studies have shown loosening or migration of the implant in up to fifty percent of the cemented femoral components of total hip replacement and in conventional acetabular components of the hip joint. Loss of fixation is particularly prevalent in young adults with up to fifty-seven percent demonstrating radiolucent lines around the metal implant and/or migration of the components and/or the need for corrective surgery within five years of the implantation. The interface between the bone and cement is generally the point of failure. Such failure is due to mechanical forces that weaken the bond between the bone and cement, and to deterioration of the cement as the result of time and other factors. Although methods have been developed to improve the bone cement and its application, the inherent limitations of bone cement are increasingly apparent in that the long term failure rates remain unacceptably high. Furthermore, there have been reports of adverse effects of the cement on the recipient of the implant including toxic reaction and sciatic nerve entrapment.
It has been suggested that the natural ingrowth of bone into porous metal surfaces of prosthetic implants might provide a clinically acceptable alternative to the use of cements and adhesives. In this respect, it is believed that bone ingrowth to irregular surfaces of an implant would produce a good bond therebetween. Such natural ingrowth of bone has been found to provide an interfacial strength between the bone and implant which is sufficient to support load bearing prosthesis. However, in order to provide long term stabilization, sufficient bone ingrowth must occur in the initial phase after implantation. In order to achieve sufficient bone ingrowth in this respect, at the present time the prosthesis has to be fixed in a stable position without movement for a period of at least six weeks in humans, and any relative motion of the prosthesis during this period either prevents or minimizes bone growth and can lead to formation of scar tissue instead of bone ingrowth. Clinical data shows that even under optimal conditions, some areas of the implant may display no bone growth.
Efforts to overcome the foregoing problems have included, for example, providing the exposed metallic portions of prosthetic devices with porous coatings, such as disclosed in U.S. Pat. No. 3,855,638 to Pilliar, providing the prosthetic device with wedge-shaped bone ingrowth materials, as shown in U.S. Pat. No. 4,536,894 to Glalante et al, and providing the prosthetic device with electrodes to influence bone growth by means of stimulating electric potential. These approaches have not overcome the problems of long term failure and long term immobilization. Other efforts, likewise unsuccessful in overcoming the foregoing problems, have included the coating of the prosthetic device with a bio-active material such as enamel or glass or glass-ceramic material, as respectively shown in U.S. Pat. Nos. 4,365,536 to Broemer et al and 4,234,972 to Hench et al. None of the previous efforts has resulted in increasing the interfacial shear strength between an implant and bone so as to significantly improve on the problem of long term failure. Further, the efforts to achieve bone ingrowth into implants have not provided sufficient bone growth to avoid the minimum six week period of immobilization following implantation in order to enhance long term stabilization.
It will be appreciated that enhancement of stabilization of implants is not only beneficial in joint replacement as discussed above but also in all dental implants, and implants designed to bridge a gap between bone ends caused by removal of a portion of the bone. The latter can be of particular importance in connection with certain humans, such as the elderly and chronic rheumatic disease patients, or other persons that have a limited or lesser than normal potential for bone regeneration.